Despite the name, there is no connection with Hundred Years' War.
150 years before the beginning of the series, the Earth Federation started their Mars colonization project, codenamed "Mars' Birthday". However, one day, due to Magnetic Ray occurring on the surface of Mars, some colonists (20% of colonists) fell ill. The Earth Federation, considering the project as a failure, abandoned the surviving colonists and covered up the reality of project, claiming that they were all wiped out by a transmittable disease; in fact, those colonists survived and were forced to survive Mars' harsh environment. Since then, Mars' situation was never told, leaving the Martian colonists to build their own nation in space.
Warring Space States Period
Some time after the Federation's colonization plan, the Advanced Generation would be used as the new calendar system. However, during the colonization period, multiple colonies and factions, most notably the Euba Alliance and Zalam went to war with each other for territory. The conflict was known as the Warring Space States Period. Eventually, the Earth Federation managed to bring Zalam and Euba to peace with the Silver Chalice Treaty. The treaty forced all colonies to disband their armed forces and prohibited any independent MS manufacturing, except for Earth Federation Forces for peacekeeping.
The Day Angel Fell/Operation Eden Phase 1
In AG 101, the colony of Angel came under attack from unknown forces and was eventually destroyed. The shocking attack took the lives of millions, including Grodek Ainoa's daughter and wife. The Earth Federation declared the event "The Day Angel Fell" and designated those invaders as a Unknown Enemy (UE for short).
War of Bat Extermination
In AG 108, another colony, Ovan, was attacked by Vagan and destroyed. The first protagonist of the series, Flit Asuno survived the attack, received the AGE Device from his dying mother and went to Nora to begin constructing a mobile suit called Gundam.
In AG 115, Nora eventually came under attack. It was then Flit activated his AGE-1 Gundam AGE-1 Normal and destroyed a Vagan's ovv-f Gafran. Vagan, learning of the Gundam, sent in their colony destroyer to destroy Nora. In the same time, all Nora colonists evacuated to the colony's ejectable port and it was pulled out by Diva (under the command of rogue captain Grodek Ainoa). While the Gundam fought the Vagan forces outside the colony, a debris held the port from being evacuated, until Commander Hendrick Bruzar, the commander of Nora's Arinston base, sacrificed himself by blowing the debris in a suicide attack.
Later, Diva traveled to Fardain. There, Flit faced the ongoing battles between the remnants of Euba and Zalam, whom still fought each other even though the war was ended. During the skirmish, Vagan's ovv-a Baqtos came into the colony and Flit fought against it, but ended up being out powered by the much stronger Baqto and being captured by Zalam's CMS-223Z Zila, thinking the AGE-1 Gundam AGE-1 Normal as an enemy trespassing in their territory. While Flit kept convincing Don Boyage, the leader of the local Zalam forces to fight UE (Unknown Enemy/Vagan), leader of Euba forces Ract Elfamel, came for payback. Again, in the middle of the battle, the Baqtos came back into Fardain. Flit managed to convince Zalam and Euba forces to cooperate against those Baqtos and he himself headed back to protect the truck taking the children from the Baqto. It was then, the AGE system created the new equipment for the Gundam, which would later be called the AGE-1T Gundam AGE-1 Titus. With this new equipment, Flit managed to defeat one unit of Baqto, while the others were defeated by Woolf Enneacle's new WMS-GEX1 G-Exes. After the ensuing battle, Zalam and Euba came to temporary truce to protect Fardain.
When Diva was leaving Fardain with several Zalam and Euba warships, they faced a Vagan's Fa Bose. Zalam and Euba MSs were no matched for Vagan's MSs while Diva had to deploy their forces to destroy those enemies units. In the middle of the battle, Woolf and Flit faced a black transformable MS, xvv-xc Zedas which once appeared during Nora's evacuation. Zedas had fast movements and tricked Zalam and Euba into hitting each other. Don Boyage, realised with Flit's words of fighting together, decided to sacrifice himself to ensure both Zalam and Euba forces worked together with Diva forces to protect humanity. His sacrifice left a lot of lessons for those forces to fight together against Vagan. Also in the battle, AGE system produced yet another new equipment, AGE-1S Gundam AGE-1 Spallow to match up with Zedas's speed. After the long battle, Vagan forces retreated with the Diva and Euba-Zalam Alliance, now under Ract's command, marched to Minsry to gather the remaining Euba and Zalam forces.
In Minsry, Earth Federation Forces 8th Space Fleet, Special Detachment forces under command of Stoller Guavaran was tasked to arrest Grodek and Diva for discipline violation. However, the 8th Fleet was under attack by Vagan. Diva and the Zalam-Euba warships protected the 8th Fleet and forced the Vagan to retreat. The 8th Fleet then let the Diva go to "protect the humanity". Once Diva arrived at Ambat, each sides launched their MSs. This time, Diva forces, Zalam and Euba forces were equipped with more effective weaponries and managed to defeat Vagan's MSs. After the initial strategy succeeded, Diva shot her new Photon Blaster Cannon installed by Mukured Madorna through blueprint from the AGE Builder under the permission of Grodek himself and destroyed the Fa Bose. At the same time, Flit faced xvb-xd Farsia piloted by Yurin L'Ciel whom was forced by Desil, who also joined the battle. When Flit's AGE-1 Spallow was heavily damaged by Desil's attacks and about to take his blow, Yurin sacrificed herself. In rage, Flit crushed Desil's Zedas, but showed him a mercy because of Desil’s tearful pleads. Flit then changed to Gundam AGE-1 Titus to open the Ambat's dock for Diva and changed back to AGE-1 Normal. Inside Ambat, Vagan's footmen fought against Diva's footmen for their last stand while Flit, with his X-Rounder ability, defeated Geera's xvm-gz Defurse. Geera retreated to his command room and revealed to Diva's forces of their real intention and who they really are. Flit, feeling angry over the death of Yurin and his mother, wanted to shoot Geera but Geera was shot by Grodek instead. Arabel, having found out his father was mortally shot, wanted to avenge his father, before being haunted by Grodek's word. After Geera threw out his last breath, the remaining forces set off a self-destruct mechanism to blow up Ambat. Diva crews managed to return to their ship in time and rendezvous with remaining forces and evacuated Ambat. After the battle, Grodek surrendered himself to EFF command for discipline. EFF, hoping to get the support from the public opinions, covered up the truth of the Diva's operation: They claimed that Diva acted independently in behalf of EFF (in fact, Diva acted independently by its own crew's wishes, not by the command of EFF) and defeated UE in Ambat. The truth about UE was revealed, but only as a nation in Mars which named Vagan and EFF didn't reveal the origin of Vagan to the public.
Operation Eden Phase 2
AG 140, 25 years since the War of Bat Extermination, Vagan rejected every peace proposals from Earth Federation and kept expanding its military. In the interval leading up to the events after War of Bat Extermination, Flit Asuno was married and appointed as Commander of EFF. Vagan started their Operation Eden, which Vagan would invade Earth. In the early stage of Operation Eden, a X-Rounder, the little brother of Desil Galette, Zeheart Galette was sent to Tordia, as Vagan's intelligence stated that Gundam was stored in Tordia. For his son's 17th birthday, Flit granted his son Asemu Asuno (the second protagonist of the series) the AGE Device, hoping his son would use it to protect his beloved ones. Right one day after Zeheart enrolled to Asemu's school, Vagan started their attack on Tordia. Asemu had to pilot his father's AGE-1 Gundam AGE-1 Normal under the recommendation of his great grandfather, Vargas Dyson. Despite having no experience with live MS battles, Asemu still managed to defeat several ovm-e Dorados with the assistance of the surviving EFF's RGE-B890 Genoace II. Later on, Vagan launched their second attack against Tordia after Zeheart found out where the Gundam AGE-1 was located, though the attack failed again, Zeheart managed to learn whom the pilot was, who was no other than Asemu. 1 year later, Vagan started to reassault Tordia one more time, having Zeheart being suspected as Vagan spy, his subordinate Daz managed to distract the military police. Vagan eliminated the Federation force which found their secret base in Tordia, but the attack failed again. Several months later, Asemu entered the military and was stationed at Diva and meeting some famous figures, such as Woolf and Millais (the new captain of Diva). Meanwhile, Ezelcant appointed Zeheart as the commander of Downes and Vagan's Earth Occupation Force for the completion of Operation Eden, the operation Ezelcant entrusted to Zeheart. Vagan launched an assault on Diva when it was enrouting to Big Ring and they also assaulted Big Ring, though the attack ended up disastrous. During the battle, Flit got an information regarding the materials Vagan got to produce mobile suits from the part of the Khronos, piloted by Desil. After analyzing the material, it was found out Solon Colony was assisting Vagan and Diva was sent to investigate. It was found out Solon was assisting Vagan, though the factory was self-destructed to prevent the Federation from finding more facts. Zeheart, whom entered Solon and met with Asemu, tried to dissuade him away from fighting, to no avail. Their meeting was found out by Romary, adding complicaton and he escaped the colony with Asemu pursuing him. Outside the colony, the remaining Magician 8 members attacked AGE-2 for the loss of their comrades, though those X-Rounders were defeated easily by Asemu and Woolf. During the voyage of Diva to Nortram, Desil led his own skirmish on Diva, even though it failed, he wasn't ordered back as Zeheart claimed that Federation might intercept their communication with Desil, revealing their camouflage to the Federation. Vagan finally completed their mission despite failing their objective: To capture Nortram. In the ensuing battle, Vagan was surprised with the completion of Photon Ring Cannon, though their Downes' Gigantes shield proved effective against the new weapon. In the battle, Woolf sacrificed himself for Asemu when fighting Desil, who was in returned killed by an enraged Asemu. Flit and the crew of Diva managed to destroy Downes propulsion systems, though it caused Downes to fall on Earth instead, before Asemu and Zeheart, while still being enemies, worked together to destroy Downes. The remaining crews of Downes evacuated in time by descending to Earth with their landing craft. Having lost their commander in the atmosphere entry, Medel Zant assumed the leadership of the remaining Earth Occupation Force on Earth. On AG 142, on the 41st anniversary of the Day of Courage (The Day Angel Fall), when the Prime Minister of Earth Federation, Froi Olfenoa, was delivering his encouragement speech on Brucia, Flit Asuno held a coup d'état against him and had a Special Task Force, under his son Asemu, to anticipate the ambush attack Vagan had prepared to assassinate Froi, whom they claimed had outlived his usefulness. The Vagan attack unit was eventually wiped out by Asemu and Froi was arrested of treachery for siding with Vagan. Soon, Flit began a purge against Federation officials that aligned with Vagan and replaced with a free government.
War on Earth
Vagan forces that landed on Earth after the battle of Nortram didn't launch any major attacks on Earth, instead, they hid themselves until the right time to strike, while also studying the Earth's terrain and environments, resulting the development of new mobile suits such as Wrozzo for underwater and land use; Gomel for underground and desert use; Reganner, MAs that are based from Defurse, a MA intended for Earth use; Danazine, the combinations of Vagan's former Dorado, Baqto and Gafran, Ghirarga etc. On AG 164, 23 years after they landed, they began their attack, resulting in battles on Earth itself, on cities and military bases such as Olivernotes and Rostroulam. The AGE-3 Gundam AGE-3 Normal is deployed, with its pilot being Kio Asuno, the third in the Asuno family line.
Last Phase of Operation Eden
A major event that occurred during the final phase of Operation Eden was the planned capture of the Gundam AGE 3 Orbital, facilitating the development of the Gundam Legilis. The Vagan supreme leader Fezarl Ezelcant reveals himself in person to Kio, and introduces him to life in the Mars Sphere. Kio's understanding of the hardships of the Vagan people improves with his time spent on the Vagan space Colony Second Moon, which is part of Ezelcant's plan to have a witness from Earth taking part in Operation Eden. Although Kio sympathizes with the Vagan people, he escapes from Second Moon with the help of Bisidian, a pirate organization headed by his father Asemu Asuno, only to encounter the Gundam Legilis and barely escape with the Gundam AGE 3 heavily damaged. The Gundam AGE FX is developed as a high performance custom suit for Kio, but after his experiences on the Mars Sphere, he shows reluctance to kill his enemies. The Battle of Luna Base sees the Gundam AGE 1 Glanza deployed, and resulted in a Federation victory with minimal casualties to the Vagan troops that occupied the base. The fabled EXA-DB is seen for the first time along with its protector Sid, and Ezelcant, with his health declining rapidly, passes on the Legilis to Zeheart Galette, at the same time making him the new leader of the Vagan. Zeheart takes on this challenge, defeats Sid, and assumes his new position. The Vagan space fortress La Gramis is deemed as a dangerous enemy asset by the Federation, and the final battle takes place just outside the Earth Sphere, with the participation of all the Vagan and Federation troops. The Digmazenon Cannon on La Gramis is fired a total of two times in this battle, causing massive losses on both sides (as Zeheart ordered the next shot to be fired on the Diva and the Gundams, regardless of friendly units that were in the way). The Diva is destroyed, but after its crew has been safely evacuated onto the Baronche, the Bisidian ship. All three Gundams escape the blast through the use of the Hyper Boost system on the Gundam AGE 2 Dark Hound, and Zeheart loses two of his valued subordinates, Fram Nara and Leil Light. Being driven off the edge, he launches in the Gundam Legilis, only to be defeated by his former friend Asemu. However, he regains his old self moments before his death, and redeems himself as a human being in front of Asemu. The battle isn't over yet, as the Vagan Gear launches and eventually its pilot Zera Gins, a clone of Ezelcant, loses control when the Vagan Gear combines with a self-repaired Sid. However, the Federation and Vagan troops unite to defeat this final enemy, and Flit Asuno is seen to finally let go of his vendetta with the Vagan.
After the war, Vagan and Earth Federation scientists were able to use data from EXA-DB and the AGE System to create the Everse System; thereby nullifying the Mars Rays and making Mars a safe refuge for the Vagan. Kio and Asemu look up at a memorial dedicated to Flit, praising him as true savior to humanity. The Gundam AGE 1 Normal had its AGE System reinstalled, and now stands as the Gundam Memorial in the Federation Capital. It is now the year AG 201, 100 years after the commencement of the Vagan-Federation war.